Fixation of prosthetic flexible tension members to relatively rigid structures has been a serious problem. A notable example is the use of artificial ligaments, such as the Leeds-Keio anterior cruciate ligament replacement in the knee. In that example, published experience with the usual means of bone fixation—drilling a hole in the tibia, inserting the tendon, and securing with a suture or pin—has included several instances of fragmentation of the polyester fibers of the prosthesis within a few months to a few years. A compression plate fixation has also been used whereby tension members are cut and the end grasped between two plates, generally textured and held together by compression screws to grasp the tension member. While this allows greater control of local stress concentration than does a simple bone-hole, in theory, it delivers extremely high shear stresses to the tension member locally, which may cause fatigue failure and breakage over the immense number of stress cycles expected to be required.
Natural tendon ends, which are living tissue, have been connected to ‘towel bar’ fixtures on artificial bones, over which they are looped and sewn. Because of the shape of tendons, generally flattened in the plane of attachment, the axis of curvature is generally parallel to the surface to which they are to be attached. To avoid intolerable protrusion dimensions into surrounding tissue structures, the radius of curvature is very small. Since the compressive stress on a tension member surface, when that tension member is looped about any rod or pulley, is directly proportional to the tension applied and inversely proportional to both the radius of curvature and the projection of contact surface perpendicular to the transmitted tension, compressive forces intolerable by the tension member may be generated.
There is thus a need in the art for an artificial force-transmitting tension member, such as an artificial tendon, that can be formed in a circular or other stable cross-sectional configuration so as to allow a medical device to be relatively narrow, flat, and oriented in the plane of the surface to which the tension member is to be attached. In this way, the radius of curvature may be made substantially larger than achievable with the ‘towel bar’ concept, and yet with only minimal protrusion into surrounding tissue structures. And, the presenting surface of the knob and of any secondary caps may be readily shaped such that minimal protrusion would present a smooth surface to the overlying skin or other tissue.